A conventional Schottky diode 10 is shown in FIG. 1. The anode 12 of the diode 10 may be any one of a variety of metals, such as aluminum. The anode 12 contacts an N-type semiconductor cathode 14, such as an epitaxial layer. An N+ layer 16 provide a contact between a voltage source and the cathode 14. Oxide regions 17 define the edges of the diode 10.
The epitaxial layer cathode 14 has a typical dopant concentration on the order of 1×1015/cm3. Typically, various other circuit elements are formed in the same epitaxial layer, such as components for a switching voltage regulator. The diode 10 may be a high power diode for connection to an external inductor in a switching voltage regulator.
When the diode 10 is forward biased, the electrons in the conduction band of the cathode 14 conduct current to the anode 12. Since the cathode is lightly doped, there is a relatively high on-resistance, such as 1 ohm where the distance between the anode 12 and the N+ layer 16 is around 2 microns.
When the diode 10 is reverse biased, a depletion region 18 forms. The depletion region 18 expands as the reverse voltage is increased until there is a breakdown. It is well known that curved edges of the depletion region 18 concentrate the electric field and thus are the weakest links for breaking down. A smaller radius of curvature of the depletion region edge lowers the breakdown voltage. Therefore, a designer of a diode junction for a high voltage application forms the junction with a large radius of curvature so that the depletion region has a correspondingly large radius of curvature. The highest breakdown voltage is achieved with a plane (flat) junction.
In FIG. 1, the curved edge 20 of the depletion region 18 will break down prior to the flat area 22 of the depletion region 18.
By increasing the dopant concentration of the cathode 14, the on-resistance will be lowered but the breakdown voltage will decrease since the depletion region 18 will not go as deep, causing the radius of curvature to decrease. Further, it may not be desirable to increase the dopant concentration in the epitaxial layer, since the layer is used for other components where the epitaxial layer dopant concentration is optimal.
What is needed is a technique to lower the on-resistance of a diode while not lowering the breakdown voltage of the diode or affecting other devices.